1. Field of the Invention
The present invention relates to a transfer device for and a transfer method of transferring a toner image carried on a surface of an image carrier to a recording sheet. The present invention relates further to an image forming apparatus using the transfer device and the transfer method.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2006-267486 discloses an image forming apparatus of this kind. This image forming apparatus forms a toner image on a surface of a drum-shaped photosensitive element by a known electrophotography process. An endless intermediate transfer belt serving as an image carrier is brought into contact with the photosensitive element, whereby a primary transfer nip is formed. In the primary transfer nip, the toner image on the photosensitive element is primarily transferred to the intermediate transfer belt. A secondary transfer roller serving as a nip forming member is brought into contact with the intermediate transfer belt, whereby a secondary transfer nip is formed. Inside a loop of the intermediate transfer belt, a secondary-transfer counter roller is disposed such that the intermediate transfer belt is interposed between the secondary-transfer counter roller and the secondary transfer roller. The secondary-transfer counter roller disposed inside the loop is grounded, while the secondary transfer roller disposed outside the loop is applied with a secondary transfer bias. This creates a secondary transfer electric field between the secondary-transfer counter roller and the secondary transfer roller, which moves electrostatically the toner image from the secondary-transfer counter roller side to the secondary transfer roller side. Then, the toner image on the intermediate transfer belt is secondarily transferred to a recording sheet which is fed into the secondary transfer nip at a timing synchronized with a transfer operation of the toner image on the intermediate transfer belt by the action of the secondary transfer electric field.
With this configuration, if a sheet, such as a Japanese sheet having a high degree of surface roughness is used as the recording sheet, a light-and-shade pattern according to the surface roughness is easily generated in an image. This light-and-shade pattern is due to the insufficient amount of toner transferred to recessed portions which brings lower image densities in the recessed portions of the sheet surface than protruding portions. For this reason, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-267486, as the secondary transfer bias, a bias composed only of a DC voltage is not used but a superimposed bias in which a DC voltage is superimposed on an AC voltage is applied. Japanese Patent Application Laid-open No. 2006-267486 reports a test result presenting that applying such a secondary transfer bias (superimposed bias) suppresses occurrence of a light-and-shade pattern as compared with a case of using a secondary transfer bias composed only of a DC voltage. Further, the same document also reports another test result. The test result implies that, as a peak-to-peak voltage Vpp of an AC component of the secondary transfer bias increases from a near zero voltage, image densities of recessed portions on the surface of a recording sheet increase gradually, whereas if the value of the Vpp is larger than twice the DC voltage, as the Vpp increases, the image densities of the recessed portions decrease. According to this test result, the densities of the recessed portions become the highest by adopting the value of the peak-to-peak voltage Vpp at a timing that is immediately before a change from a tendency to increase the image densities of the recessed portions on the surface of the recording sheet to a tendency to decreases the image densities of the recessed portions in a process of gradually increasing the peak-to-peak voltage Vpp, that is, the value of the Vpp which is about twice the DC voltage is used.
However, the inventors of the present invention have found through the tests that, in a case of focusing only on transferring a sufficient amount of toner into the recessed portions of the surface of the recording sheet, that is, in a case of applying the value of the Vpp which is about twice the DC voltage, a good result is not obtained. That is, it is found that in order to obtain a sufficient image density in the recessed portions, the value of the Vpp must be more than that value. Specifically, according to the tests of the inventors of the present invention, unlike the test results disclosed in Japanese Patent Application Laid-open No. 2006-267486, even when the value of the Vpp was larger than twice the DC voltage, the image densities of the recessed portions increased as the value of the Vpp increased. However, when the value of the Vpp was made relatively large, in a particularly deep recessed portion of the recessed portions, a discharge occurred between the bottom of the particularly deep recessed portion and the intermediate transfer belt, resulting in white dots, dot-shaped lacks, in the image. If a number of white dots are generated and the white dots are connected long, it may appear the image densities of the recessed portions fall significantly short of the mark. However, a shortage in the image densities and the white dots are completely different phenomena. Specifically, the shortage in the image densities is caused when an insufficient amount of toner is transferred from the intermediate transfer belt or the like into the recessed portions of the surface the recording sheet, whereas the white dotes are generated when the toner oppositely charged due to the discharge cannot be transferred into the recessed portions. Even in an image in which a large number of white dots were generated so that it appeared as if a significant storage in the image densities of the recessed portions was caused, a higher image density than that obtainable in a case that the Vpp was set at twice the DC voltage was obtained in a relatively shallow recessed portion of the recessed portions. In other words, it is considered that even in the tests disclosed in Japanese Patent Application Laid-open No. 2006-267486, if the white dots and the shortage in the image densities are precisely distinguished, similarly to the tests conducted by the inventors of the present invention, even in a high potential area where the potential is larger than twice the DC voltage, the image densities increase as the value of the Vpp increases. Further, according to the tests of the inventors of the present invention, in order to obtain allowable levels of image densities at the recessed portions, the value of the Vpp needs to be four times larger than the DC voltage.
Moreover, it has also been found that, for example, even when the value of the Vpp is made larger than four times the DC voltage, if a sheet having a relatively large thickness or a relatively high degree of surface roughness is used as the recording sheet, sufficient image densities may not be obtained at the recessed portions on the surface of the recording sheet.
For this reason, the inventors of the present invention conducted tests using recording sheets having a large thickness and a high degree of surface roughness, and, as a result, found that, in order to obtain sufficient image densities at the recessed portions of the surface of the recording sheet, the value of the Vpp should be made larger than six times the DC voltage.
Until now, the image forming apparatus having a configuration in which the toner image is transferred to the recording sheet at the secondary transfer nip has been described. However, even in a configuration in which a toner image is transferred from a photosensitive element to a recording sheet at a transfer nip formed by contact of the photosensitive element and a transfer roller or the like, the same problems may occur.